Senate Democrats' campaign committee on Thursday began canceling plans for television ads in Louisiana's major markets to help Sen. Mary Landrieu's runoff campaign against Republican Bill Cassidy, making her re-election bid an even steeper challenge. The Democratic Senatorial Campaign Committee initially had reserved $1.8 million in television advertising for after Tuesday's first round of voting. On Thursday, the DSCC dumped the bookings in the state's big cities. A spokesman for the debt-laden committee said officials still back Landrieu, who has twice before prevailed during runoff elections, and would keep an eye on the race. "Mary Landrieu is a proven runoff...