Only a couple of months ago Sen. George McGovern (D-S.D.) seemed a sure loser in his bid for reelection, reeling under sledgehammer blows from a potent right wing coalition and trailing badly in the polls.

Sen. John Culver (D-Iowa) was faring no better under a similar pummeling from the right, running well behind amd making no visible progress.

Sen. Frank Church (D-Idaho) held only a shaky lead, hardly an auspicious start for a four-term veteran and powerful committee chairman, and he was bracing for a Republican tidal wave.

Now McGovern and Culver have slogged their way back into the running, with polls even showing a slight lead for Culver, and Church faces no worse than even odds. Both sides in all three states agree, with varying degrees of edginess, that their races are probably still to be decided.

What linked the three races from the start was the intensity of the effort by the National Conservative Political Action Committee (NCPAC), antiabortionists and other attack troops of the New Right to purge the Senate of three of its most prominent liberals -- a task made easier by the fact that the incumbents' states are generally conservative.

What appears to link them now -- in their new role as possible election-night cliffhangers -- is the comparable intensity with which the incumbents have fought back, challenging the New Right head-on and attempting to link their Republican rivals to the backlash that seems to be developing from the New Right's heavy-hitting tactics.

Once accused of succumbing to Eastern effete-ism by virtue of their long stays in Washington, their liberal voting records and their infrequent visits to their west-of-the-Mississippi states, they have shed their coats, rolled up their sleeves and come out swinging like righteous avengers of savaged reputations.

Take McGovern, for instance.

After two years of being called everything from "baby killer" to stooge of Fidel Castro, the 58-year-old three-term senator and onetime Democratic presidential nominee got mad and grabbed the offensive away from his Republican challenger, Rep. James Abdnor (R-S.D.)

Abdnor, 57, an amiable conservative who has represented rural western South Dakota in the House since 1973, had kept his campaign formally independent of the New Right effort but clearly had been profiting from its attacks on McGovern.

So when McGovern found in a poll that the harsh rhetoric of the New Right attacks actually helped him, he started running some of their claims in his own commercials. A favorite of the McGovernites is a reported claim by John T. (Terry) Dolan, director of NCPAC, that the group could elect Mickey Mouse if it wanted to. The claim is now a centerpiece of McGovern's television ads.

Abdnor has expressed relief that NCPAC has moved on to other states and said recently that "I'd tell them to get the heck out" if they tried to come back.

McGovern has also pulled rank on some of the Moral Majority preachers who are railing against him by trotting out a Methodist bishop who warns against an "extremist" invasion of the state. He's also hit at Abdnor with newspaper ads about an "oil spill in Abdnor's office," referring to his pro-oil legislative record and campaign contributions from the oil industry, and with radio ads on Abdnor's record (10 seconds of silence).

McGovern, who was running as much as 26 percentage points behind in Republican polls earlier this summer, is still behind in GOP voter surveys but closed the gap and moved ahead in a United Press International poll earlier this month.

"It's trench warfare, brutal on both sides," said a Republican Senatorial Campaign Committee official.

In Iowa, freshman Sen. Culver, has climbed from a 17-point deficit in the independent Iowa Poll last summer to a 5-point lead in the latest version of the poll earlier this month, although aides to Rep. Charles Grassley, his Republican challenger, contend the momentum has switched again and belongs to Grassley. An aide to Culver describes the race as "tight as a tick."

Perhaps more than any of his nervous liberal colleagues, the 48-year-old Culver has refused to trim sails to survive the conservative winds that are reputedly buffeting the land. Instead, the onetime Harvard fullback and Marine Corps. infantry officer has simply charged ahead, banking on a belief that Iowans will respond favorably to his consistency.

Culver, too, has ranted against the New Right and hit back hard at Grassley, a 43-year-old farmer and three-term House member whose conservatism is as unyielding as Culver's liberalism. Slashing at Grassley's record just as Grassley and his New Right boosters have slashed at his record, Culver has succeeded -- to some extent, at least -- in seizing the offensive. He has dissected Grassley's voting record on issues ranging from the B1 bomber to Social Security, doing enough damage to provoke charges of smear tactics from the Republicans.

In Idaho, 55-year-old Church, chairman of the Senate Foreign Relations Committee, might well be in more serious difficulty were it not for the political missteps of his challenger, conservative three-term Rep. Steve Symms.

In the latest of these, Symms was accidentally caught by network television cameras planting a question at a rally about whether Church, as head of the committee that investigated the Central Intelligence Agency five years ago, shared some responsibility for the highly publicized killing of a CIA agent whose cover was blown. The incident got prominent play on television and on the front pages of Idaho newspapers.

Still, Church is clinging to no more than a 2 1/2 percent lead in the last independent statewide poll, down slightly from where he was during the summer and up only marginally from an earlier 1-point spread. Both sides claim to be moving ahead in the stretch, but independent observers say the race is probably so close it could go either way.

With an especially vigorous local chapter that predated the national effort, NCPAC spent $219,000 in Idaho by Sept. 30, the largest expenditure from the $840,000 it had allocated for its six targeted Senate races as of that date (including Democratic Sens. Alan Cranston of California, Birch Bayh of Indiana and Thomas F. Eagleton of Missouri as well as Church, Culver and McGovern).

Moreover, the conservatism of Church's Rocky Mountain constituency, is probably deeper than that of Iowa or South Dakota, a point underscored by the fact that his vote for the Panama Canal treaties is still such a raging issue in the state.

But at the same time, Church campaign aides claim Symms' espousal of the "sagebrush rebellion" against federal control of western lands is beginning to backfire as Church pounds away at the alternatives: costly state maintenance of the lands or their desecration through development. He is banking heavily on Idaho being as conservationist as it is conservative.