On Tuesday, it was a 4,500-mile round-trip to Boise and Milwaukee to raise campaign funds for two Republican senators facing reelection in 1986. On Wednesday, it was a tete-a-tete in the Oval Office which helped persuade a reluctant former Vermont Gov. Richard Snelling to run against Sen. Patrick J. Leahy (D-Vt.) next year. On Thursday, it was a fund-raising dinner for the Republican Governors Association, the opening gun in a concerted drive to improve the GOP's strength in the state capitols.

In these and other less-publicized ways last week, President Reagan went about fulfilling his role as the day-in, day-out leader of the Republican Party. As one who often has been critical of other aspects of Reagan's stewardship, I want to salute the man who, more than any other president I have ever covered, pays his dues and unselfishly aids the growth of his political party.

From Eisenhower through Carter, the presidents in office used their parties when it suited their needs and stiff-armed them when they felt like it. By contrast, Reagan consistently has soldiered away for the Republican Party, in good times and bad. As Rhodes Cook of Congressional Quarterly pointed out, about the last thing Reagan did before he went in for cancer surgery last summer was tape a television ad for the GOP candidate in a Texas special election. One of the first things he did when he got out was speak at a California Republican fund-raiser.

Edward J. Rollins, who left recently as White House political director, said Reagan made more than two dozen campaign and fund- raising appearances for the party and its candidates in 1983-84, despite his own reelection campaign. "He cut TV spots for more than 300 candidates," Rollins recalled.

Mitchell E. niels Jr., Rollins' successor, saw Reagan's "extraordinary effort" in 1983-84 as executive director of the Republican Senatorial Campaign Committee, but said he still was not prepared for Reagan's "total readiness" to shoulder political chores. So far this year, he has done events for all four major national Republican committees, for six senators with 1986 races and for both states with 1985 gubernatorial races. He has cut TV spots for elections as obscure as the Honolulu city council recall contest, entertained a group of Democratic converts (with another yet to come in) and held the first of several White House receptions and briefings for GOP activists.

What drives the president to this self-appointed task? The answer from former press secretary Lyn Nofziger and from almost everyone else who has known Reagan over the years is that when he chose the Republican Party, after spending his first 50 years of life with the opposition, he signed an oath in blood.

"He really believes in the party," Nofziger said, "and he wants to see it succeed. Long before anyone approached him about being a candidate himself, he was doing party chores, and he just never has stopped."

"He has a unique relationship with his party," agreed Charles T. Manatt, who was California Democratic chairman during Reagan's governorship and Democratic National chairman during the first term of his presidency. "In his second term as governor, he was out working for legislative candidates, and now he's doing the same thing in his second term as president. He just doesn't quit."

As long as he has been active in the GOP, Reagan has preached the importance of party unity, and he practices it in his own work. Of all the Republican senators running for reelection in 1986, the first to ask for and get a Reagan endorsement tape for radio and TV use was Bob Packwood of Oregon, perhaps Reagan's most caustic Republican critic during the first term.

Reagan had no more difficulty finding areas of agreement to emphasize in plugging for maverick Packwood's reelection than he did in finding common ground last Tuesday with such staunch conservatives as Sens. Steve Symms of Idaho and Robert W. Kasten Jr. of Wisconsin. By historical standards, as Congressional Quarterly noted, Reagan did not show great coattail strength in his landslide victories of 1980 and 1984. His help did not turn the tide for Republicans in the special elections this year in either Texas or Hawaii, and despite his plans to campaign actively in 1986, he may not be a major factor in the mid-term election either.

But in the long-term, he has provided entree for other GOP candidates to constituencies that had been strongly Democratic before 1980: Southerners, evangelicals, blue-collar skilled workers, pink-collar office workers and, most important for the future, young people and first-time voters.

More than that, he has added both muscle and definition to the Republican Party. As Secretary of Labor Bill Brock, a former Republican National Chairman, said last week, "The biggest advantage we have is that we have a party to which people can rally and respond. We have a strong leader and a clear sense of direction, and the Democrats have neither."

The Republicans have a president who really works at politics. And it makes a difference.