Why did Ralph Yarbourgh vote for Civil Rights?

[Fuzzy Says: "Abolish NPR!"]

And did it cost him re-election in 1970?

He was never up for re-election, because Lloyd Bentsen successfully primaried him from the right. His support for civil rights was a strong factor, however, with Bentsen accusing him of being in favor of busing and other unpopular practices. Bentsen also attacked Yarborough's record on the Vietnam War as insufficiently hawkish.

Bentsen in 1970 was recruited by John Connally to settle a score with Yarborough (the two hated each other).  Bentsen received a degree of support from non-Tory National Democrats in Texas because of their fear that they would lose the Senate seat to George Bush in what promised to be a strong, well-funded campaign that was conspicuously backed by the Nixon Administration. 

You'd think that such scheming would put Connally on the outs with Nixon, but this wasn't the case.  Nixon always looked beyond party to an "ideological majority".  Nixon didn't care much about domestic issues, but he cared a great deal about international issues.  In Charting the Candidates 1972, Ronald Van Doren noted that a clue to Nixon's style of management was his dividing line between domestic issues (which Nixon was generally apathetic about) and the international issues (which were his, and only his, concern).  Van Doren explained that Nixon's interest in domestic issues was confined to the way in which a domestic issue affected the international issues.  Bentsen, like Bush, was an improvement in foreign policy over Yarborough; he was not likely to support anything like the Cooper-Church or McGovern-Hatfield amendments on Vietnam.

[Fuzzy Says: "Abolish NPR!"]

Bentsen in 1970 was recruited by John Connally to settle a score with Yarborough (the two hated each other).

As was also the case between Yarborough and Connally's mentor, LBJ. These divisions within the Texas Democratic Party were precisely why JFK went to Texas in November 1963...

EDIT: I didn't see Yankee's post.

Yarborough was, indeed, a national liberal.

[Fuzzy Says: "Abolish NPR!"]

Bentsen in 1970 was recruited by John Connally to settle a score with Yarborough (the two hated each other).

As was also the case between Yarborough and Connally's mentor, LBJ. These divisions within the Texas Democratic Party were precisely why JFK went to Texas in November 1963...

EDIT: I didn't see Yankee's post.

Yarborough was, indeed, a national liberal.

But a populist one, too!

I remember reading about his 1964 Senate race and just ridiculing the very notion of George Bush being elected in Texas . “Aw look, Prescott Bush, sent his boy down here and gonna buy him a Senate seat!” (paraphrased)

IIRC, Yarborough ran about 12 points behind LBJ and almost lost.  Who do you think LBJ voted for in the GE?  To say nothing about John Connally and the Texas Democratic delegation in Congress?

[Fuzzy Says: "Abolish NPR!"]

If Connally voted for Bush he kept it quiet.

The whole purpose of JFK's trip to Dallas was to help settle a feud between Connally and Yarborough.  Connally wanted to dump Yarborough and his candidate was Rep Joseph "Fighting Joe" Kilgore, a Democrat from the Austin area who was less liberal than Yarborough, but still a National Democrat.  Kilgore wasn't O. C. Fisher or Omar Burleson, conservatives who were crypto-Republicans; he was a moderate Democrat who was more of a get-along-go-along type and who LBJ liked.

The dispute was somewhat about ideology, but not as much as one would think.  Just like Harry Truman's support of the conservative George Smathers over the liberal Claude Pepper in 1950, this was more personal than philosophical.