Voters wait in a long line to cast their ballots in Denver, Nov. 7, 2006

Polls have begun closing in the United States in elections that will determine who controls Congress for the next two years.

Preliminary, unofficial results are coming in after a day of relatively smooth voting, although it was too early to declare the results of most races. Problems with electronic polling equipment forced officials to extend voting hours in some places.

The F.B.I. is also investigating complaints that attempts were made in Virginia to keep some people away from voting.

Pre-election surveys showed voters were focusing on on the war in Iraq and national issues, including terrorism and corruption, as they headed to vote. The polls also showed strong support for opposition Democrats who hope to regain control of the House of Representatives for the first time in 12 years.

But surveys also indicated several key races are closer than they were earlier, and Republican leaders are claiming they have the momentum in the election.

The elections also include voting for governors in 36 states and ballot questions on issues ranging from abortion and homosexual marriage to stem cell research.

President Bush and First Lady Laura Bush walk out of the Crawford Fire Station after casting their vote

President Bush and First Lady Laura Bush voted in their hometown of Crawford, Texas, early Tuesday and later returned to Washington.

Republicans currently hold a majority in both the House of Representatives and the Senate.

Both parties were concentrating their efforts on getting their supporters to the polls.

On Monday, the president campaigned in the southern states of Florida, Arkansas and Texas. Mr. Bush urged all to vote Republican if they value their own security and if they want low taxes. Although the president is not running for office, he has campaigned on behalf of Republican candidates.

Democrats have appealed to voters to force a change in Washington.

Members of the House of Representatives serve two-year terms and are all elected at the same time. Senators serve six-year terms, with one-third of the 100 seats contested every two years.