President Trump following a Republican's loss in a special Pennsylvania congressional election is encouraging voters to elect candidates who will support his agenda in the midterms.

A Trump campaign email with the subject line "I need Allies" sent Saturday includes a survey for respondents to describe what qualities are most important to them in a candidate.

"2018 is our chance to elect TRUMP ALLIES to the House and Senate who will give you the agenda you voted for. No more obstruction! We want what we voted for!" the email, which is signed by Trump, states.

"But it cannot happen unless we put up GREAT candidates who stand by our values and prove to America that they have the fire to fight to Make America Great Again!" he continues.

Despite early successes, Trump-backed candidates have not fared well in a couple of recent elections.

Democrat Conor Lamb narrowly defeated Republican Rick Saccone in a special election for a Pennsylvania congressional seat earlier this month. Trump appeared at a rally for Saccone days before the election.

Trump also backed Republican Alabama Senate candidate Roy Moore, despite allegations he had inappropriate relationships with teenage girls when he was in his 30s. Moore lost to Democrat Doug Jones in a December special election.

Saturday's email blast came shortly after Trump roiled lawmakers when he threatened to veto a spending bill that passed both Republican-controlled chambers of Congress. He ultimately signed the bill, citing the military funding it provides.

He has also expressed frustration with Democrats, claiming they are obstructing the confirmation process for his nominees for various federal posts.