US President Donald Trump has announced that Republicans will hold their first-ever national convention ahead of the midterm elections, an unusual event aimed at boosting turnout in races that will decide whether the party maintains control of Congress.

The convention will be held in Dallas on 9 and 10 September.

Although both major parties traditionally hold blockbuster conventions during presidential campaigns, Trump has long floated the idea of a similar gathering this year to focus voters' attention on a sprawling collection of House and Senate races.

The party that holds the White House in the first midterm elections after a presidential election usually loses congressional seats.

Republican sources said that the event is expected to be held around the second week of September, possibly at the American Airlines Center.

Battle for Texas

Republicans face a crucial test in Texas over whether they can continue to dominate politics in the state.

Trump dealt a stunning blow to longtime incumbent Republican Senator John Cornyn by endorsing his opponent, Texas Attorney General Ken Paxton.

Paxton easily defeated Cornyn in the Republican primary election. This has set up a November battle between Paxton and Democrat James Talarico.

Democrats see the scandal-tarred Paxton as vulnerable and are pouring money into the race in hopes of finally breaking Republicans' grip on the state.

Trump wants to use the convention to rally his Make America Great Again faithful to vote in November, the sources said.

Voter turnout in midterm elections is historically much lower than in presidential elections.