Bernie Sanders told supporters Thursday he will work with Clinton to transform the Democratic Party and beat Donald Trump, but he is still not conceding.

"The major political task that we face in the next five months is to make certain that Donald Trump is defeated and defeated badly. And I personally intend to begin my role in that process in a very short period of time," said Sanders.

Sanders was not included in a Bloomberg poll on potential vice presidential candidates, released Wednesday. According to a CNN source, Sanders in not being considered for that role.

The poll did show that Democratic voters' top choice for Clinton vice president is Massachusetts Sen. Elizabeth Warren.

In the poll, 35 percent said Warren would be the best pick, ahead of New Jersey Sen. Cory Booker at 17 percent. However, Booker says he is not being vetted for the job.

House Speaker Paul Ryan says he will continue to speak out in defense of conservative principles, despite a warning from likely Republican nominee Trump that GOP congressional leaders should be quiet.

Despite his differences with Trump, including Trump's proposed ban on Muslims entering the country, Ryan says he has no plans to rescind his endorsement.

"He's a different kind of candidate. It's a different kind of year, I'm going to be myself and stick up for my principles and not just our party's principles but our country's," said Ryan.

Representative Fred Upton, a 30-year House veteran, says his decision to not back Trump would mark the first time in his three-decade congressional career that he was not endorsing the GOP presidential candidate.