Sen. Mark KirkMark Steven KirkThis week: Trump heads to Capitol Hill Trump attending Senate GOP lunch Tuesday High stakes as Trump heads to Hill MORE (R-Ill.) begins 2015 with a $2 million war chest, as he heads into what's likely to be a tough reelection fight.

The top-targeted GOP senator raised more than $600,000 from October through December 2014, according to Crain's Chicago Business.

That's a solid haul for a candidate not up for reelection during a fundraising quarter when others were scrambling for last-minute cash, though he (and his opponents) will have to raise much more than that each quarter to stay competitive.

Kirk's most likely opponent, Rep. Tammy DuckworthLadda (Tammy) Tammy DuckworthFourth Senate Dem calls for Nielsen to resign over family separation policy Dem senators move to halt potential US troop withdrawal from S. Korea Dems accuse Interior of holding up key grants MORE (D-Ill.), heads into 2015 with more than $1 million in the bank after raising $460,000 over the same time period.