Former governor Terry Sanford has turned out to be a lousy underdog in the North Carolina Senate race. Every time somebody takes a new poll, he's in front.

Rep. Harry M. Reid (D) has no business being ahead in the Senate race in Nevada, where the leading import, aside from gamblers, is Republicans. But his opponent has stumbled out of the starting gate, and Reid has been steady.

Rep. John B. Breaux (D) shouldn't be able to close the gap in the Louisiana Senate race at the same time he's being heavily outspent. But that's just what's happening.

All three Democrats are running for open Senate seats. There is a bumper crop of seven open seats this year, the product of three Democratic and four Republican retirements. For reasons that have to do less with national issues and moods, and more with the zigs of personality or the zags of primaries, they're all in better shape at the onset of summer than they had expected.

So are a handful of other Democratic Senate hopefuls around the country -- and so, by extension, is the whole Democratic effort to recapture control of the Senate, the grand prize of this curiously themeless midterm election.

Democrats need a net gain of four seats to turn around their current 47-to-53 deficit, end a six-year spell as the minority party in the Senate, and build momentum toward 1988.

With the primary season mostly complete, and with many of the 34 fall campaigns now engaged, a beaming Sen. George J. Mitchell (D-Maine), chairman of the Democratic Senatorial Campaign Committee, proclaimed late last week: "We're ahead in the polls in six seats now held by Republicans Florida, North Carolina, Maryland, Idaho, South Dakota, Nevada , we're within striking distance in many others, and all our incumbents are in good shape."

Apply the standard discount for partisan embellishment, and Mitchell's assessment is one that even some Republicans buy. "I definitely don't like what I'm seeing out there," said David Spear, a spokesman for the Republican Majority Fund, one of many political action committees and partisan institutions working to keep the Senate in GOP hands.

It would be tidy to peg these Democratic gains to a national trend or a burning issue. But with the exception of tax revision, it's all quiet on the issue front these days -- and tax revision has turned into such a happy bipartisan crusade that it's hard to imagine it hurting, or helping, either party in November.

The death of SALT II? Some Democrats say they'll try to reawaken the long-dormant arms control constituency, but as long as there's a chance for a post-election U.S.-Soviet summit, it's hard to see that issue congealing.

Pocketbook issues? Yes, there are sectors of the economy in deep trouble, and trade issues will play for the Democrats in some areas. But in the main, the economy is still humming. "I can't remember any other time, five months out from a midterm election, things looking so rosy for the party in power," said pollster Robert Teeter.

Mid-May surveys taken for the Republican National Committee by GOP pollster Richard Wirthlin make Teeter's point:

Eighty-one percent of the electorate say they think the national economy is as strong or stronger than it was a year ago; 75 percent think it will be as strong or stronger a year from now.

Eighty-two percent of the electorate say their economic circumstances have stayed the same or improved in the past year; 91 percent say they think their family's economic situation will stay the same or improve in the next year.

So what's the source of the Democratic glow? Simple: Party strategists don't think the 1986 Senate races will be decided by the national mood. They say they think the country is in a kind of political interregnum between the Reagan Era and some as-yet-undefined post-Reagan period. Voters are without their moorings. They've already said thank-you to Reagan; they're not quite sure what they want next. In their uncertainty, they'll focus more on character, less on ideology.

"The messenger is the message," Democratic pollsters Peter Hart and Geoffrey Garin write of the political dynamics of 1986. "Voters are focusing on the candidates' character to a greater extent than in the past three elections, and the primary role of issues in 1986 will be to provide insights into a candidate's personal qualities and priorities."

This means:

Democrats have new hope in North Dakota because the man who was their second choice to take on Sen. Mark Andrews, Tax Commissioner Kent Conrad, turns out to be a hard-charging, fast-on-his-feet challenger. Conrad is telling Dakota farmers that if they want to register their hurt over falling land values and price supports: "I'm your messenger."

Democrats are better off than they expected to be in Pennsylvania because the winner of a tight Democratic primary, Rep. Bob Edgar, has good "geography." He represents the Republican Philadelphia suburbs and a small slice of the city, and that's exactly where the freshman incumbent, Sen. Arlen Specter (R), ran up the big numbers he needed in 1980 for a razor-thin statewide win.

Democrats are better off than they thought they'd be in North Carolina because the Republican primary was so nasty that the winner, Rep. James T. Broyhill, will carry some unexpected baggage into the fall.

Democrats are upbeat about their prospects in Idaho because they unearthed a picture of incumbent Sen. Steve Symms (R) shaking hands some years back with Libyan leader Muammar Qaddafi, and it's getting wide circulation in the state these days.

With the exception of Idaho, where popular retiring Gov. John V. Evans (D) always figured to be a strong challenger, these others are states that weren't on many Democratic watch lists six months ago.

This political roundup of spring breaks is not all one-sided, of course. In Colorado, the good news for Republicans is that Rep. Ken Kramer (R) just saved himself two months of hard campaigning, hundreds of thousands of dollars and no end of intraparty grief when both of his primary opponents dropped out of contention Saturday after they narrowly missed the 20 percent vote at the state GOP convention that would have automatically qualified them for the primary ballot. On the other hand, many Colorado Democrats consider Kramer their easiest foe.

The best news for the GOP has come in California, where Rep. Edwin V.W. Zschau (R) broke smartly out of a big primary field last week and is ready to take on Sen. Alan Cranston (D), who turns 72 next week. Some Democrats are already fretting that Cranston will gather so much national Democratic money defending his seat -- he's a peerless fund-raiser -- that party resources will be drained from tight races elsewhere.

For tight races, there's no better place to look than the seven open-seat races -- Arizona, Colorado, Louisiana, Maryland, Missouri, Nevada and North Carolina.

Some of the developments this spring in the hottest four:

Louisiana: There's been a lively ping and pong of television ads this past week that essentially encapsulates the race between Breaux and Republican Rep. W. Henson Moore for for the seat that Sen. Russell B. Long (D) is giving up.

The thrust from Moore: An ad opens with a blank television screen, then it flashes the number 1,083. Men and women in the street try to figure out what the number means. "The number of times I gave up smoking?" "How many years it will take me to graduate from college?" Wrong. It's the number of votes John Breaux missed since he came to Congress. "Is that a record?" somebody asks. End of spot.

The parry from Breaux: Same opening, same 1,083 on the screen. Same man-in-the-street bewilderment. New kicker: It's the number of jobs Louisiana loses every 10 days as a result of Republican policies in Washington.

By raising questions about the way his opponent has conducted himself in office, Moore is implicitly trying to tie Breaux to his old boss, Gov. Edwin W. Edwards (D), whose popularity remains low despite an acquittal on a federal conspiracy charge. Breaux is trying to focus on his own effectiveness (his ads say that he's passed 18 bills in Congress, Moore has passed none), and to tie the state's oil and gas woes around the Republicans. Moore is likely to have a 3-to-1 or better edge in television money in the last weeks of the campaign.

North Carolina: The way to beat Terry Sanford is "to call him a liberal and make it stick," said Scott Cottington, political director of the National Republican Senatorial Committee. That's just what Broyhill will do. But Sanford strategists say they're ready. "In the primary, we ran ads about him seconding JFK's nomination in 1960 and fighting for racial justice, and we established them as acts of courage," said his pollster Harrison Hickman.

Broyhill has a couple of other problems. In his primary, he spent so much time denouncing negative ads that he'll have to find a way to carry the attack against Sanford without seeming hypocritical. And he'll have to shake the image of being a tool of the nuclear energy industry and the utilities -- a charge his Republican opponent leveled at him in the closing days of a bitter primary.

Missouri: Lt. Gov. Harriett Woods (D) narrowly missed winning a Senate seat in 1982 against incumbent Sen. John C. Danforth, and now she's back for another try, this time for the vacancy created by Democratic Sen. Thomas F. Eagleton's retirement.

She'll be far better funded than she was in 1982, and she's latched onto the trade and farm crisis issues: "If we can refinance loans for South America, we can do it for farmers in Missouri."

Her opponent is former governor Christopher S. (Kit) Bond, who went on the air last month with a round of ads that implied Woods was trying to exploit the woes of farmers. One spot quotes a farmer saying he gets a little tired of politicians who jump on the farm bandwagon around election time. Bond will stay on major issues, talking about the strength of the national economy and how Reagan needs a Republican Senate to make his last two years his best.

Nevada: Reid lost a bid for the Senate to Republican Paul Laxalt by fewer than 1,000 votes in 1974, and when Laxalt announced his retirement, Reid jumped for the seat. He will play the race as a contest between a Democrat who puts his state's interests first against a Republican who puts his own interests first.

The Republican is former representative Jim Santini, who switched parties last year, announced that he didn't do it to run for Senate, then became a Senate candidate. Laxalt and Republican National Committee Chairman Frank Fahrenkopf reportedly had a hand in keeping Rep. Barbara F. Vucanovich (R) out of the race to clear the way for Santini. The whole orchestration has left a sour taste in some GOP mouths, and Santini's fund-raising is off to a slow start.

There is another potential problem for Santini in Nevada: It's one of three states under study to be the site of a national nuclear waste repository -- an unpopular proposal. Both Santini and Reid are against it, but Reid will ask the voters why, with all of Nevada's Republican clout in Washington, it has gotten no relief, while East Coast sites have been removed from consideration