PARTY REALIGNMENT, that will o' the wisp of American politics, is again flickering in pundits' eyes. Noted politicians are changing parties, and it's natural to look to see if voters are too. Last fall you could hear talk of a permanent Republican majority, and you could find some basis for it in poll results: more voters said they were Republicans and fewer said they were Democrats than in the previous 40 years. But the elections produced the split-level result that is getting to be a habit: a Republican president, a narrowly divided Senate, a Democratic House. Earlier this winter, when President Reagan's popularity ratings were high, more people were saying they were Republicans. But this spring, in tandem with the drooping of the economy, Mr. Reagan's ratings and Republican prospects nationally seem to have declined.

Yet through all these vibrations in political popularity, there is evidence of a permanent change in one important segment of the electorate -- white southerners. From the Carolinas to Texas, identification with the Democrats dropped precipitously last year and has stayed at historically low levels this year. Most, though not all, of the recent party switchers, from would-be Texas Gov. Kent Hance to seven Louisiana legislators, are in the South, and the national Republican party has targeted three southern states (plus Pennsylvania) in its $100,000 drive to encourage voters to switch party registration.

The Republicans are hoping for the breakthrough that has eluded them since the 1950s. Southern states started voting Republican for president in those years, but they have declined to vote Republican in the large majority of congressional, state and local contests. The GOP's chances now seem as good as they've ever been. A key test willbe in the special election to be held soon to replace a conservative east Texas Democrat who's becoming a federal judge. Republicans are making a major effort there. If they can win, they believe they can, in contests with no incumbents running, capture something like 40 seats now held by Democrats -- and with them control of the House.

Of course few southern House Democrats are changing parties this year, and only a few will vacate their seats in 1986; the Republicans are going to have to fight to make gains, and they will be handicapped, as they have since the Eisenhower days, by a paucity of candidates with government experience and good political instincts. But they hope that once the Republican Party reaches a critical mass, both in number of politicians willing to run on their ticket and ordinary people willing to vote a straight Republican ticket, it will break through in the South and nationally to majority status. Such a breakthrough seems possible to an extent that almost no one expected even 12 months ago.