WASHINGTON - Independent US Senator Bernie Sanders of Vermont on Thursday announced he would seek the 2016 Democratic nomination for president in a bid likely to pressure Hillary Clinton from the left and challenge her on financial issues.

Sanders, a self-described socialist and one of the most outspoken liberals in Congress, faces a difficult fight against Clinton, the presumptive party frontrunner.

"The people at the top are grabbing all the new wealth and income for themselves, and the rest of America is being squeezed and left behind," Sanders said in an email to supporters declaring his candidacy.

"The middle class in America is at a tipping point. It will not last another generation if we don&39;t boldly change course now," he wrote.

Sanders, 73, is an ardent supporter of left-leaning policies such as expanding Social Security and raising the federal minimum wage. Now in his second Senate term and previously a member of the US House of Representatives, he caucuses with Democrats even though he was elected as an independent.

He has cultivated a following among some American liberals for campaigns on income inequality and social issues.

In 2010, Sanders stood on the Senate floor for more than eight hours lecturing about corporate greed and criticizing Wall Street as he sought to delay a tax bill that would extend cuts initially enacted by former President George W. Bush.

He will likely struggle to compete against the fundraising might and name recognition of Clinton, a former US secretary of state and first lady. But after running for office and losing numerous times in the 1970s, Sanders has become known for doggedly fighting uphill battles.

Sanders could be embraced by activists as a substitute for US Senator Elizabeth Warren of Massachusetts, a darling of the left who has said she is not running after liberals spent months urging her to get in the race.

Warren has made clear that she wants the party&39;s nominee to take progressive stances on battling Wall Street and bolstering the middle class.

Sanders may be able to use his entry into the primaries to nudge Clinton to take more liberal positions than she might have otherwise.

Clinton is under similar pressure from former Maryland Governor Martin O&39;Malley. He has not yet said whether he will run, but he has staked out liberal stances on issues such as free trade agreements being negotiated by the White House.