There is no simple answer to this question, partly because it is complex and partly because senators and voters are not put to the test on abortion in the same ways.
First, there is a degree of complexity to this question. Senators are explicitly members of their party. Voters are not. There are some voters who vote Republican not because they are loyal to the party but because the party is more in line with their views on abortion. If such voters disagree with the Republicans on abortion, are they being less loyal to the party or are they being loyal to their views on the issue of abortion?
Second, voters do not have to (or do not get to) vote on as many issues regarding abortion. Voters, therefore, are not put to the test in the same way that senators are. Senators have to take specific stands on specific laws. Voters can simply state their opinions without having to actually vote on sticky issues having to do with abortion.
All that said, it is possible to argue that senators are more loyal and partisan. This is because they are likely to have to satisfy the most hardline voters of their parties in primary elections. A senator who votes against their party on abortion will have a hard time being reelected because the minority of voters who care most about abortion are likely to vote in the primaries. Meanwhile, there can be a mass of voters who are more conflicted and moderate on abortion and are, therefore, less partisan than a Todd Akin or a Richard Mourdock.