The Republican party chief is appealing for unity, but some of Donald Trump's critics still want no part of him.

FOX's Jared Halpern has more from Washington:

It could take time for some wounds to heal, concedes RNC chairman Reince Preibus:

(Preibus) "This is what happens when you go through, not jut a difficult primary, but unprecedented."

Priebus says a strong top of the ticket is needed to win House and Senate contests. Senate Majority Leader Mitch McConnell will support the presumptive nominee, but adds Donald Trump has the obligation to unite the party.

Easier said than done for some Senate colleagues, Nebraska Republican Ben Sasse penning an open-letter asking why voters should be limited to just Trump and Hillary Clinton.

In Washington, Jared Halpern, FOX News.