WASHINGTON — This year Senate Republicans are threatening filibusters to block more legislation than ever before, a pattern that's rooted in — and could increase — the pettiness and dysfunction in Congress. The trend has been evolving for 30 years. The reasons behind it are too complex to pin on one party. But it has been especially pronounced since the Democrats' razor-thin win in last year's election, giving them effectively a 51-49 Senate majority, and the Republicans' exile to the minority. Seven months into the current two-year term, the Senate has held 42 "cloture" votes aimed at shutting off extended...