Actually Congress has been relinquishing power to the executive Office for over 20 years . . For at least that long, Partisan politics has been the tool of Congress, either obstructing a president or when all are of one party, legislating from the Oval Office with Congress' approval.
Bush 43, actually, was the first to have his office called the "Imperial Presidency".
Obama has just taken full advantage of that with first a compliant Congress and then a split one which has enabled him to use "Legislative Gridlock" as every excuse to create or modify passed Law.
And those who run Congress are very happy with the resulting turmoil. Because of the partisan politics on both sides, there's no one to hold their feet to the fire and have Congress do it's constitutional duties . .
And they certainly haven't.