From Missouri Secretary of State Jason Kander (D), a candidate for the U.S. Senate in 2016, via Twitter:

Jason Kander ‏@JasonKander
Yesterday terrorists shot and killed 14 innocent people. Today @RoyBlunt voted to let suspected terrorists buy guns. Unbelievable. 4:16 PM – 3 Dec 2015

From Igor Volsky at Think Progress:

igorvolsky ‏@igorvolsky
Sen. Blunt received $755,816 in expenditures from the NRA, so he voted against today’s background check measure. 5:44 PM – 3 Dec 2015

There is a difference.

From Senator Claire McCaskill’s (D) office:

McCaskill Office ‏@McCaskillOffice
Claire: Why would we let a suspected terrorist legally buy an assault weapon? A majority of Americans want loopholes closed #StopGunViolence 4:58 PM – 3 Dec 2015

Good question.

The vote in the United States Senate:

U.S. Senate Roll Call Votes 114th Congress – 1st Session

Vote Summary

Question: On the Motion (Motion to Waive All Applicable Budgetary Discipline Re: Feinstein Amdt. No. 2910 )

Vote Number: 319 Vote Date: December 3, 2015, 04:29 PM
Required For Majority: 3/5 Vote Result: Motion Rejected
Amendment Number: S.Amdt. 2910 to S.Amdt. 2874 to H.R. 3762 (Restoring Americans’ Healthcare Freedom Reconciliation Act of 2015)

Statement of Purpose: To increase public safety by permitting the Attorney General to deny the transfer of firearms or the issuance of firearms and explosives licenses to known or suspected dangerous terrorists.

Vote Counts:
YEAs 45
NAYs 54
Not Voting 1

[….]

Blunt (R-MO), Nay
McCaskill (D-MO), Yea

[emphasis added]

Let that sink in for a minute.