Washington — While political tides roll in and out during the year before an election, the tide so far for United States Senate races has been all one direction - in favor of Republicans keeping the upper house.
It has been a ''first the good news, then the good news'' situation for the GOP Senate campaign, which has 19 Republican seats to defend. Only a year ago a dozen looked shaky, but with 1984 only two weeks away, that list has dwindled to four clearly endangered seats.
Even in those races the GOP is finding some cause for hope. Two Republicans who seemed doomed by low approval ratings have begun to show comeback signs. Sen. Jesse A. Helms of North Carolina, with aid from donations from his national base of New Right and antiabortion supporters, has closed in on his unannounced opponent, Gov. James B. Hunt Jr., a Democrat. Senator Helms has cut the early Hunt lead from 20 to 10 points or less in polling.
Meanwhile, Sen. Roger W. Jepsen of Iowa, also thought to be in deep trouble, pulled even with his opposition, US Rep. Tom Harkin (D), in a poll just published by the Des Moines Register. Only two months ago Mr. Harkin had an 11 -point lead.
The other two endangered GOP seats, in Texas and Tennessee, are both open in 1984 with the retirement of John Tower and Howard H. Baker Jr. Democrats have the inside track in Tennessee, with Rep. Albert Gore Jr. as their candidate, but the Texas race is a free-for-all, with primaries still ahead to pick the contenders for both parties.
Overall, the Republicans seem to be getting most of the breaks in the early days of the campaign. With a strong boost from an improving economy and help from a soaring approval rating for the Republican President, their incumbents suddenly look less beatable. For now, the GOP has what is jokingly called ''big mo'' (momentum) on its side.
The first tangible sign was the election last month in Washington State of Republican Dan Evans to fill the unexpired term of the late Sen. Henry M. Jackson (D). It did not hurt that the special election came just after the Reagan administration's hugely popular invasion of Grenada.
Such helpful coincidences have been following the GOP around of late, as in the experience of Sen. Gordon J. Humphrey, a right-leaning Republican from New Hampshire. A first-term lawmaker and former airline pilot listed as vulnerable, Senator Humphrey happened to be visiting Lebanon the weekend the truck bomb exploded in the US Marine base.
His eyewitness story made big news in his home state at a time when the Lebanon attack was virtually the only news story.
But on the day after the Oct. 23 bombing there was a second story. It happened that Rep. Norman E. D'Amours, a moderate Democrat, was announcing he would run for the Senate in New Hampshire. The timing was unfortunate, concedes a D'Amours spokesman, but plans had been made and the news media notified, so it was too late to change.
Other Republican advantages are far from happenstance. As in past years, the Republicans have far more money to throw into Senate campaigns than do the Democrats. Senator Humphrey, for example, has been amassing a campaign chest for two years which his office reports at more than $300,000, compared with $175,000 for Democrat D'Amours.
Sen. Richard G. Lugar of Indiana, chairman of the National Republican Senatorial Committee, expects to have $50 million to spend, enough to promise every GOP candidate the maximum party aid allowed by law. The Democratic committee will probably have $5 million.
Both parties have been hurt because star candidates have declined to run in key Senate races. Governors particularly have been reluctant to jump into the arduous campaign for an arduous job in Washington.
But the gubernatorial mood has spared some Republicans some very tough races. In Mississippi, popular Gov. William F. Winter, the Democrat most likely to unseat Republican Sen. Thad Cochran, has just decided he would rather be chancellor of the University of Mississippi. In Virginia, Gov. Charles S. Robb and in Colorado Gov. Richard D. Lamm have both chosen to stay in the statehouse rather than challenge Republican senators.
Republicans have had their recruiting woes, too, not to mention losing majority leader Baker of Tennessee and Armed Services Committee chairman Tower of Texas, who are retiring next year.
But overall, Republicans are upbeat. ''Sure, I feel better about it,'' Senator Lugar told a breakfast meeting of reporters in assessing the campaign, which he predicts will end up with a 52- or 53-seat total for the GOP in the Senate.
The one fly in the GOP's campaign ointment is foreign events.
Much of the recent jump in Reagan popularity is said to stem from his Grenada action, and that could fade as rapidly as it appeared, especially if the US becomes deeply embroiled in fighting in Lebanon.
''It's one of those volatile issues that can go against the President one week and back for him in another,'' says J. Brian Atwood, director of the Democratic Senatorial Campaign Committee.
Republicans back in their home states for the congressional break are waving warning signals over US involvement in Lebanon. But Senator Lugar appears unruffled. ''Domestic economic issues are uppermost for two-thirds of Americans, '' he said this week.
''There's a great concern about Lebanon,'' he said, but he predicted that by election time, that hot spot will no longer be a crucial campaign issue.
Meanwhile, if unemployment stays down, Republican hopes will stay high, according to the campaign committee chairman.
The committee is still holding out a 50-50 chance of winning the six seats needed to retake the Senate, which now has 55 Republicans and 45 Democrats.
But among other Democrats the word going around is that they are writing off 1984 and setting their sights on the Senate in '86.