Party identity at the time Mathias began his political career was not based on ideology to the same degree it is today. On average, Republicans in Congress were more "conservative", as that word is understood in modern American political discourse, than the average Democrat. But both parties were more ideologically diverse than they now are.
Mathias's record fits the modern American definition of "liberal". However, he is quoted in his New York Times obituary discussing the root meanings of words such as "conservative":
“I’m not all that liberal,” he told The Washington Post in 1974. “In fact, in some respects I’m conservative. A while ago I introduced a bill preserving the guarantees of the Bill of Rights by prohibiting warrantless wiretaps. I suppose they’ll say it’s another liberal effort, but it’s as conservative as you can get. It’s conserving the Constitution.”
He was correct that one who conserves is a conservative. But granting that high a priority to civil liberties is not called "conservative" in our modern political lexicon. I wrote about that subject here.
Had Mathias remained in Congress in this century, chances are he would have switched to the Democratic Party, as Pennsylvania Senator Arlen Specter did last year. And, of course, conservative Democrats have moved in the other direction.
There is much anguish in our current political commentary, about the supposed evils of ideological parties. But I don't see it that way.
For one thing, there is a kind of truth-in-advertising advantage. When today's voter chooses a Republican or a Democrat, they have a better idea of what they're getting, than would have been the case a few years ago. During the 1970s and '80s, Mathias and Senator Jesse Helms of North Carolina would each have appeared on a ballot under the Republican label, even though they disagreed on major issues. Same thing on the Democratic side between, for example, Mississippi's John Stennis and Minnesota's Hubert Humphrey.
There are still gaps between fellow partisans such as Republicans Olympia Snowe of Maine and Jim DeMint of South Carolina, and Democrats Barbara Boxer of California and Ben Nelson of Nebraska. But those gaps are narrower, as evidenced by the Senate's party-line health care vote last month.
Another issue is that party allegiances in the past were, to a great extent, based on ethnicity and, by extension, religion. The Republican Party was the party of Protestants, including, especially before Franklin Roosevelt's presidency, African Americans. On the other hand, the Democrats were seen as the party of Jewish and Roman Catholics voters.
As a practical matter, intermarriage and secularization have combined to weaken modern Americans' ethnic and religious identities. That was bound to lead to changes in the basis of partisan identities. Also, perhaps in part as a result of those larger societal trends, partisan identities are also weaker. To identify one's self as independent has become a sort of badge of honor.
Additionally, dividing the body politic along ethnic lines contributed to ethnic stereotyping and segregated political activity. Is that the golden age to which critics of ideological parties want to return?