Julian Castro, 44, the grandson of a Mexican immigrant who would be the first Hispanic elected president, served as secretary of Housing and Urban Development under former President Barack Obama and has long been viewed as a rising star in the party.
Democrats say they are determined to obtain more information about meetings by Trump and Secretary of State Mike Pompeo with North Korean Leader Kim Jong Un, worried that Trump is so eager to make a "great deal" that he will give Kim too much.
If Democrats take control of one house, the Republicans will struggle to get legislation passed. In the 2010 midterms, during the regime of President Barack Obama, Republicans gained control of the Senate and obstructed legislation moved by Democrats.
Americans, as well as the rest of the world, are keenly watching the elections, whose results could impact President Donald Trump’s powers in matters of the legislature as well as in getting his nominees confirmed to judicial and other positions.
Democrats held a wide lead among blacks, Latinos, women, independents and voters between the ages of 18 and 34, the poll showed. Republicans led among voters ages 50 to 64, men, and whites, according to the poll.
The conviction and guilty plea on Tuesday of two former members of Trump's inner circle put a spotlight on the Democratic argument that Republicans have failed to check the excesses of corruption under Trump - but that voters could do it for them.
On Ingraham's radio show on Wednesday, fellow Oklahoman and Pruitt ally Republican Senator James Inhofe stopped short of calling for his resignation but said if the stream of scandals does not stop, he would be forced to ask him to step down.