Former U.S. representative Kent R. Hance, a folksy, conservative West Texan who narrowly lost the Democratic Senate primary last year, will switch to the Republican Party Friday to pave the way for a gubernatorial bid in 1986, political sources said today.

The switch would mark one of the biggest coups in the nationwide Republican campaign to translate President Reagan's landslide reelection victory last November into party-building at state and local levels.

Hance, 42, is expected to announce the move and hint at a possible gubernatorial candidacy at a Washington news conference where he is to be joined by Sen. Phil Gramm (R-Tex), who made a similar change of parties two years ago.

Hance and Gramm were leading House Democratic "Boll Weevils" during President Reagan's first two years in office. Hance was Democratic cosponsor of the administration's landmark 1981 tax-cut legislation, and Gramm cosponsored its 1981 budget-cut bill.

Several Texas Democratic leaders expressed chagrin at Hance's impending defection, conceding that he will make a formidable challenger to incumbent Gov. Mark White (D), whose approval ratings have sagged in the last year.

"It's a real coup for the Republicans," said George Christian, a lobbyist and Democratic strategist who was press secretary to President Lyndon B. Johnson.

"It shows what a strain it is for conservative Texas Democrats to stay in the party. The national party keeps digging itself into a deeper hole, and it takes real valor to stay with them," Christian said.

Since last November, eight local, elected officials in Texas have switched to the Republican Party. GOP leaders here said they hope Hance's move will trigger a "handful" of defections by Democratic state legislators after the current state legislative session. Democrats outnumber Republicans, 2 to 1, in both chambers, but the gap has narrowed in recent years.

"This is a real cornerstone of the realignment that has been taking place in Texas among voters and will now begin to take place among officeholders," said Jim Francis, a GOP strategist from Dallas who, along with Gramm, was most influential in bringing Hance over the fence.

Christian agreed, saying, "The talk about whether this is going to become a two-party state is old hat. We're already there. The question now is which party will have the upper hand."

Other Democrats took a less dire view. "Hance is just the latest in a long line of boys who thought the Republican Party was a shortcut to success but who's going to find out he's wrong," one elected Democrat said, asking not to be identified.

Hance lost his bid for the Democratic Senate nomination by 1,400 votes last year to former state senator Lloyd Doggett, a liberal defeated by Gramm last November.

If Hance seeks the governorship in 1986, he might face a primary fight in his new party from, among others, Rep. Tom Loeffler (R-Tex.), who has signaled interest recently.

Texas Democratic Chairman Bob Slagle said late last month that Hance had repeatedly assured him that he did not plan to change "parties or wives."

Before the news of the impending switch had spread this morning, Gov. White said Hance assured him Wednesday that he had "no plans to switch."

Hance was unavailable for comment