By Steve Holland

HOUSTON (Reuters) - President Barack Obama sharply criticized what he called the least productive U.S. Congress in modern history on Wednesday in a fund-raising speech that he used to try to energize Democrats to vote in November congressional elections.

Obama blasted Republicans in the U.S. Senate for blocking a Democratic-supported bill earlier in the day aimed at addressing a gap in pay between male and female workers. Republicans argued that pay discrimination is already illegal.

Obama also cited Republicans' refusal to agree to an immigration overhaul and an increase in the minimum wage as examples of what he called obstruction by his political opponents.

"This has become the least productive Congress in modern history, recent memory. And that's by objective measures - just basic activity," Obama said.

Obama was speaking to 60 contributors at the luxurious Houston home of trial lawyer John Eddie Williams. The event raised money for both the Democratic Senatorial Campaign Committee and the Democratic Congressional Campaign Committee. Tickets for the event ranged between $16,200 and $64,800.

He also attended an earlier event benefiting the Democratic National Committee.

Obama is struggling to get Democratic voters excited about mid-term elections in November that will set the tone for the remainder of his presidency.

Usually the party that controls the White House during mid-term elections loses seats, and Republicans believe they will be able to build on their majority in the U.S. House of Representatives and may oust Democrats from control of the Senate.

The president's political standing is also ringing alarm bells. After the disastrous rollout of his signature healthcare law last October, his job approval rating has fallen below 50 percent.

Obama said Republican "obstruction" this year may be a good political strategy if Democrats do not vote in the mid-terms. Democrats are active in presidential campaign years, he said, but "we have this congenital disease, which is in mid-term elections, we don't vote at the same rates."

"We need you to take these mid-terms as seriously as any presidential election that you've ever been involved in," said Obama.

Obama is spending two days in Texas. On Thursday he will speak in Austin at the presidential library of Lyndon B. Johnson to mark the 50th anniversary of the passage of the landmark Civil Rights Act.

(Editing by Ken Wills)