First his future father-in-law switched parties, becoming a Republican before running again for county commissioner. Then state Rep. Derrick Seaver's fiancee defected from the Democrats - to vote for her father in a 2003 primary.

Then it was Seaver's turn to leave the Democrats, the first Ohio lawmaker in 22 years to make the switch but one in a long history of politicians around the country parting ways with their party.

Lawmakers who make the switch do it for their conscience, their short-term gain or their long-term ambition, say observers of the jump from one side of the aisle to the other.

''There's always this tension between, are voters electing a person to represent them in the Legislature, or are voters electing a member of a party?'' said Antoine Yoshinaka, a University of California-Riverside political scientist who studies lawmakers who switch parties.

''For a lot of these folks who switch parties, they will argue, 'My constituents elected me to represent them, therefore I'm doing what's in their best interests,''' he said.

In Georgia, three Democratic representatives switched parties after the GOP gained control of the House Nov. 2 for the first time in 130 years.

''As a minority member lobbyists find your company far less enchanting than they used to,'' said Charles Bullock, a University of Georgia political scientist.

In Louisiana, U.S. Rep. Rodney Alexander switched his party affiliation from Democrat to Republican in August, drawing the ire of his former state party boss. Republicans control the U.S. House and Senate.

The switch was an ''underhanded move motivated entirely by short-term political opportunism and personal gain'' said Mike Skinner, chairman of the Louisiana Democratic Party.

Alexander's chief of staff brushed off such criticism, saying Alexander had long voted along conservative lines and supported many of President Bush's initiatives.

''The name-calling is unfortunate,'' said Royal Alexander, who is not related to the congressman.

People who switch parties are often successful when running for re-election, said Christian Grose, a political science professor at Wisconsin's Lawrence University who studies the phenomenon.

But not always. New York saw an exception in 2000 when a 71-year-old former librarian named Regina Seltzer defeated the incumbent congressman, Michael Forbes, who had switched from the GOP to run as a Democrat.

Seltzer, who won by 35 votes in the primary before losing the general election, said she didn't dislike Forbes but was disappointed the Democratic Party was supporting someone who didn't share its values.

''I was appalled that the Democratic party not only seduced him to switch but supported him so wholly,'' said Seltzer, 75, now a practicing lawyer. ''I ran because nobody else would do it.''

Before Seaver, the only precedent in recent Ohio history was the temporary defection to the GOP in 1982 by state Sen. Morris Jackson, a Cleveland Democrat. Jackson agreed to become the 17th Senate Republican - giving the GOP control of the chamber - in exchange for becoming Senate president. But the deal fell through after about two weeks and he returned to the Democrats.

Such switches can be minor political coups but with no impact on most voters, worth noting only for the bragging rights.

In rare cases, the move swings control of a chamber to another party. That was the case when U.S. Sen. Jim Jeffords, a Vermont Republican, became an independent and gave Democrats 19-month control of the Senate in 2001.

Jeffords cited his discontent with the conservative direction the Republican Party was taking under President Bush.

Seaver, the youngest lawmaker in Ohio history when he was elected at 18 in 2000, opposes abortion and the death penalty, favors gun rights and voted against a penny sales tax increase last year supported by many Republicans.

''I plan on holding those same values and beliefs this term and hopefully future terms that I've held in the past,'' Seaver said in making his announcement last week.

Time will tell, said Grose.

''There's a lot of evidence that party switchers become more conservative or more liberal depending on which party they switch into,'' he said.

Andrew Welsh-Huggins covers the statehouse for the Ohio Associated Press.