At a midsummer cookout of the Orange County Republican Party here, the name tags came in three colors: blue for longtime Republicans, green for recent converts, red for Democrats.

Normally, ecumenism does not get dished out alongside the hot dogs and beans at such party shindigs. But the loyal Republicans of this 350-year-old community outside Durham have some missionary work to do this summer.

With a frank eye on public relations, the Republican National Committee has launched "Operation Open Door," an off-season voter registration program designed to convert 100,000 Democrats into Republicans in four states -- North Carolina, Pennsylvania, Florida and Louisiana -- by Aug. 15.

To meet their county quotas, local party activists in each of the states are hosting "switcher" bashes, setting up registration desks at shopping malls and sometimes trekking door-to-door.

At one level, the exercise seems a bit like trying to rearrange a desert one grain of sand at a time. Despite recent GOP political successes, this is still, by party registration, an overwhelmingly Democratic nation. Last November, the edge was 36.6 million Democrats to 21.9 million Republicans in the 28 states that register voters by party. In North Carolina, the Democratic edge is an even more lopsided 2 1/2 to 1.

If Republicans can pull off a 49-state presidential landslide in the teeth of such a deficit, what's the point of investing $750,000 trying to switch a mere 100,000 voters?

Not for the numbers themselves, party strategists say, but for what the numbers deliver: post-election momentum to coax along the grass-roots political realignment they believe is taking place.

Party leaders are trying to capitalize on surveys that show an increasing number of voters identifying themselves as Republicans, even if they are still registered as Democrats or Independents. GOP officials believe that by persuading voters to change their registration, the Republican party can achieve parity with the Democrats, long the dominant party in the country.

"The most important thing I'll do this year is recruit candidates to run for three legislative seats we ought to be able to pick up in 1986," explained Barbara Boyce, chairwoman of the Mecklenburg County (Charlotte) GOP. "When you're trying to get somebody to run and you can say, 'Look at how our registration is growing,' it makes it a whole lot easier."

To support local party efforts, the RNC is sending out 1.3 million pieces of mail to Democrats in the four states that the Reagan campaign identified as GOP supporters. It is also airing $350,000 worth of advertisements -- a 30-second "positive/negative" spot that invites Democrats to leave the party of Walter F. Mondale and Edward M. Kennedy and join the party of President Reagan.

"It's smart politics," said Paul Tully, a Mondale campaign staff member who is now heading Kennedy's political action committee. "They're trying to connect Reagan with Republican ism , which is a good move because a lot of Democrats tend to see him as being above partisanship. They're also warming up in states that all have tough Senate races next year."

Despite that tip of the hat from the other side, Operation Open Door has had its share of early disappointments and doubters. For starters, the RNC wanted its letter to go out over Reagan's signature, but that proposal was scotched by Max L. Friedersdorf, White House congressional liaison, who worried that Democratic congressmen would get ornery about supporting the president's legislative program if he was busy plucking their electoral base from under them.

Instead, the letters have been signed by Republican governors or senators. To make up for the loss of Reagan's signature, the RNC increased the mail volume 30 percent.

As for the doubters, they argue that, when it comes to "realigning moments," 1985 is not what it is cracked up to be.

Conservative strategist Kevin Phillips, for instance, contends that realignment has been going on for 16 years at the presidential level, and its "big early juice" is spent.

But Phillips' views would not go far at the Big Barn in Hillsborough, where 80 people gathered to fuss about politics.

"Ten years ago, if we got 30 people at a party function in the heat of a campaign, that would be one hellacious turnout," marveled Monroe Knight, a lifelong Republican. "This is really amazing."

There were six green name tags (recent converts) on the hot dog line, eight red tags (Democrats) and no shortage of perspectives on party identification. Three samples:

*"The Democrats have become the party of the minority groups -- that was clear from the San Francisco convention," said Norman Haithcock, chairman of the nonpartisan county board of education, who switched this year. "They don't make decisions any more based on what's good for everybody."

*"The current national Democratic leaders aren't going to chase me out of my party," said Jack Martin, an engineer who explained he showed up only for the food and fellowship. "They'll change," he predicted. When? "When they've spent all the government's money."

*"I'm going to switch tomorrow," announced John Cross, a retiree and lifelong Democrat who recently moved here from New York. " Sen. Jesse Helms is a barrier, but I really think this state needs a two-party system."

Making the civics-book pitch for a two-party system was Robert Touchton, 27, a former Helms campaign worker who heads a six-person staff for the North Carolina effort. In a pep talk, he spoke of how the "demagogic" Democratic-controlled legislature had gutted Republican Gov. James G. Martin's tax-reduction program, curbed his appointive powers and rammed through a bill that would place state and local elections on an odd-year cycle, so North Carolina Democrats would not have to run on tickets headed by national Democrats. That measure will go before voters next year.

He spoke, too, of the need to capture control of at least one chamber of the legislature by 1990 so that Republicans can hold their own in reapportionment.

Republicans now have 50 of the 170 seats in the two chambers of the legislature, double the number they controlled before last November and well above the 20 percent average for all southern legislatures. "What you're seeing in the South is not a sudden realignment, but an evolutionary process," said Hastings Wyman Jr., publisher of the Southern Political Report.

Tar Heel Republicans have been unable to persuade any sitting Democratic legislators to switch parties, but they have collected a smattering of switchers among city council members, county commissioners, former legislators, small-town mayors and ministers. Their biggest catch of the year is former Charlotte mayor Eddie Knox, who narrowly lost the Democratic gubernatorial primary last year.

"The pattern is pretty clear," said state Democratic Chairman Wade Smith. "The losers switch."

Smith called the GOP program "crazy. They're not getting any votes they don't already have, and they're sure doing a good job of motivating my party." He predicted a big Democratic recovery in the state next year.

Other Democrats see other silver linings. "If we lose some of the reactionary Democrats, it may help us get our act together as a party," said Gary Pearce, who managed former Democratic governor James B. Hunt's unsuccessful Senate race against Helms last year.

"What hurt Hunt was the perception that he was trying to have it both ways, that he was appealing to blacks but trying to hide it," Pearce continued. "If the party changes, we may be able to bridge some of those gaps more honestly."

But while Democrats seem to have to strain a bit to unearth prospective advantages to a more ideological alignment of local parties in the South, Republicans are practically salivating at the prospect. "If all the southern conserva- tives jump over to our party," said William I. Greener, the political director of the Republican National Committee, "what you've got left is a Democratic Party that will nominate candidates who can't win in November."