A Republican has been elected governor of Texas for the first time in a century, an event that could begin a new era in the state's politics as surely as it ended one.

Bill Clements, a wealthy oil driller who committed a vast fortune to his Republican candidacy, defeated Democratic Attorney General John Hill by 20,000 votes out of 2.2 million cast.

It was perhaps the most stunning upset of Tuesday's gubernatorial elections, and it left the Republicans in charge of the statehouse in the nation's third most populous state.

Sen. John Tower, who until Clements' victory was Texas' only Republican ever elected statewide, also survived a bitter challenge from Democratic Rep. Bob Krueger for a fourth Senate term.

"I looks like Texas is now a two party state," said Billie Carr, a liberal Demoratic national committeewoman from Houston.

What others saw in Clements' election, however, were the effect of particularly low voter turnout at traditional Democratic boxes, and a growing willingness among conservative Texas Democrats to vote Republican.

Beyond the state's boundaries, the election of Clements could polish the image of presidential aspirant John B. Connally. A former Texas governor and a Democrat-turned-Republican, himself, Connally campaigned strongly for Clements, especially in Houston, where Clements beat Hill by 27,000 votes - more than his statewide margin.

"It makes Connally's transfer over responsible, makes Connally look good," says Republican pollster V. Lance Tarrance of Houston.

Republicans also picked up two House seats, according to unofficial but almost complete returns. With Republicans Tom Loeffler picking up Krueger's seat and Ron Paul defeating Rep. Bob Gammage in Houston, the state's House delegation will now be 20 Democrats and four Republicans.

The losses by Gammage, Hill and Krueger may have been as much Democratic failings as Republican successes - for all three were apparently mortally set back by low voter turnout among Democrats.

Gammage said he was hurt by a light turnout among black voters in Houston.

In surrounding Harris County, Hill won 95 of every 100 votes cast in low-income areas, but it was offset in affluent white areas that went 75 percent for Clements. White turnout was nearly 60 percent and black turnout was 25 percent.

In South Texas, both Hill and Krueger were hampered by poor turnout among Mexican Americans, but they also saw Republican inroads in traditionally Democratic rural areas. Returns showed that large numbers of people cast their votes for Republicans in the Senate and gubernatorial Democrats as attorney general, lieu-races, then "returned home" to elect tenant governor, state legislators and Congressmen.

Clements made heavy use of his own wealth, spending at least $6 million and perhaps as much as $10 million, to wage a classic conservative, jingoistic Texas campaign against spending, liberals and Yankees.

He painted Hill a liberal, big spender - a charge Hill had successfully weathered in his victorious primary election campaign against incumbent Democratic Gov. Dolph Briscoe. Ronald Reagan had campaigned for him, as did ex-President Gerald Ford.

Clements will have to work with a Democratic legislature, on the sensitive and crucial matter of redistricting after the 1980 census, in which Republican hope to avoid being gerrymandered out of existence.

Clements, a former assistant secretary of defense under Ford, campaigned heavily against the Carter administration, portraying Hill as "one of them." Carter barely carried Texas in 1976.