LAST YEAR the Republicans were crowing about party-switchers. This year it's the Democrats. The Republicans bragged that Tampa Mayor Bob Martinez, Wayne County Executive William Lucas and former representative Kent Hance switched parties and were running for governor in Florida, Michigan and Texas. Now all three are opposed in their primaries and are underdogs in November. Sen. Phil Gramm (R-Tex.) hoped to change the political balance of rural southern districts by electing former Aggie quarterback Edd Hargett in the Texas First Congressional District special election last summer; he lost by a hair and is not running this fall.

Contrast that, the Democrats say, with this. Longtime California state Sen. Milton Marks, after beating Democrats for years, turned Democrat and was promptly elected caucus chairman. David Johnson, a Hector, Minn., farmer who was a Reagan delegate at the Republican National Convention in Dallas, is now a Democratic-Farmer-Labor candidate against a young Republican star, Rep. Vin Weber. The Democrats, who led the Republicans only 37 percent to 35 in Gallup's party affiliation poll a year ago, rebounded to a 40 to 33 percent edge by the end of the year.

Has the tide turned? Don't count on it. Republicans may not be running many candidates in districts like the Texas First, but they still have the potential of winning them. The Republican label is no longer the kiss of death, as it was until recently in local races in the rural South. As for the Democratic party-switchers, they don't exactly come from Reagan country. Mr. Marks is from San Francisco, which went 67 percent for Walter Mondale; Mr. Johnson is from the part of Minnesota where Mr. Mondale served as coordinator for the 1948 Hubert Humphrey Senate campaign. As for party identification, the Democratic percentage for the year is the lowest in the last 50 years, and the Republican percentage the highest since 1946.

The Democrats remain, as they were in 1984, competitive in statewide and local races almost everywhere, even in heavy Reagan constituencies. But the Republicans remain competitive in more places and for more offices than they have been since at least the Eisenhower and maybe the Coolidge years. The party switches and poll results the Democrats are crowing about are evidence of minor fluctuations in a political environment in which both parties are competitive and capable of winning.

The danger for the Democrats is not that they will get swamped, as some Republicans hoped they would in 1985. The danger for the Democrats is that they will become convinced that happy days are here again and conclude that all they have to do to win elections is to hold up the Democratic banner and wait for the voters to salute. In the current political environment, wise Democrats and wise Republicans both understand that neither party has a built-in advantage. Candidates have to win votes the old-fashioned way: they have to earn them.