There is only one party in Washington DC, the UniParty.  The professional political class support themselves beyond all other ideological factors. These are modern realities.

President Obama is in a very tenuous position this week as a nation has been stunned by the targeting of police officers.  Indeed, there is a solid argument to be made that President Obama created the antagonism toward law enforcement with various speeches and cursory accusations of unequal treatment.
As a modern social justice president who embraces radical extremist groups like Black Lives Matter, and as an executive community activist who has used the cabinet offices of his administration to leverage social change, President Obama has embraced many divisive entities and expressed ideological alignment with their cause.
The gunman in Dallas who targeted white police officers, specifically because of the color of their skin, is just one visible consequence from a President who rails against a system of law and order.  Obama’s ideology is more comfortable amid the chaos. 
Having to face the consequences of that chaos, President Obama is in a very vulnerable position.  He was also facing the very real possibility to going to Dallas and finding a less than hospitable community enraged with him as they are left to deal with the horrific consequences of his antagonisms.
Enter George W. Bush to present himself as a noble shield.

Valerie Jarrett provides the cover – President Barack Obama and former President George W. Bush on Tuesday will speak at an interfaith memorial service in Dallas for five police officers slain late last week, as well as meet with the victims’ families.
The President will visit the Texas city at the request of Dallas Mayor Mike Rawlings, White House Press Secretary Josh Earnest said in a statement Sunday afternoon. On Sunday evening the White House announced that Bush would join his successor at the memorial service in Texas.
Vice President Joe Biden will also attend the service at the Morton H. Meyerson Symphony Center, as will former first lady Laura Bush. (read more)

More than likely Obama will have to strike some of the paragraphs from his intended speech delivery.  Specifically the demand to disarm all Americans.
The former President standing next to the current President demanding gun removal could be a bridge too far even for the progressive, and always professionally political, Dubya.
However, it is President Obama giving the main speech, so there will be more than a moderate dose of misdirection and obfuscation as POTUS talks of the scary military guns he defines as “weapons of mass destruction” that must be kept out of the hands of the folks, Americans.
You can take an activist out of Chicago, but…. well, you know.
Simultaneous to this event, the professional political class will use Bernie Sanders endorsement of Hillary Clinton to fill the news cycles and column inches with pro-Democrat, cum-by-ah unity et al, talking points and sound bites.  These constructed MSM events are insufferably all too predictable now.
The unknown question surrounds the previous plan for the Democrats to follow-up Obama’s speechifying parseltongue with another takeover of the House of Representatives chamber for a sit in.
In the past several days the unleashed masses of the Black Lives Matter have pushed the envelope a little too far.  The tone and sense of the electorate may not be favorable right now to seeing Democrats further push the divisive rhetoric.   In addition they don’t want to dilute the media time for Sanders/Clinton.  Thus, they may have to wait a few days.