Sen. Chris Van Hollen announced Thursday that he'll challenge Chuck Schumer for majority leader.Why it matters: Van Hollen's bid gives grassroots activists nearly three weeks to pressure Senate Democrats before they cast secret ballots in the Dec. 1 leadership election.The attempt to oust Schumer (D-N.Y.) underscores that deep divisions are plaguing the Democratic Party despite its overwhelming midterm success.Van Hollen (D-Md.)